Various methods are known for removing liquid from mixtures of solids and liquid. In cases where solids consist primarily of compressible granular, fibrous or cellular material and the liquid is water, equipment selected for these applications have included belt presses, centrifuges, screw presses and vacuum filters.
There are technical and economic reasons for selecting a particular technology. Technical reasons for selecting a belt press include the ability to operate continuously; the ability to produce a dry press cake with a target specification for % solids (or % moisture equal to 100%-% solids); and the ability to retain a high proportion of the incoming solids in the press cake.
Numerous examples of belt presses are provided in the prior art—for example, U.S. Pat. Nos. 7,381,329 (Moss), 6,454,102 (Thompson), 5,592,874 (Blauhut), 5,545,333 (Louden et al.) and 4,681,033 (Crandall et al.). In general, a belt press has a frame with an upper belt and a lower belt. Both upper and lower belts are typically liquid permeable [see U.S. Pat. Nos. 6,454,102 (Thompson), 5,592,874 (Blauhut), 5,545,333 (Louden et al.), 5,109,764 (Kappel et al.), 4,986,910 (Uyama et al.), 4,961,862 (Janecek), 4,879,034 (Bastgen), 4,836,100 (Johnson et al.), 4,681,033 (Crandall et al.), 4,181,616 (Bahr), 4,147,101 (Heissenberger et al.), and 3,942,433 (Wohlfarter)], although U.S. Pat. No. 5,022,989 (Put) suggests a non-permeable upper belt combined with a permeable lower belt is more advantageous. U.S. Pat. No. 4,986,910 (Uyama et al.) discusses the limitation of twin permeable filter fabrics for dewatering sludges.
Belt presses can employ several stages at which different techniques are used to remove the liquid from a mixture of solids and liquid. The use of these different stages improves filtering efficiency throughout the belt filter press.
In a typical belt press with multiple stages, the mixture initially enters a gravity screening zone of the belt press where free water drains through the lower permeable belt by gravity. The gravity screening zone is used primarily to remove free water. At the end of the gravity screening zone, the mixture could be near 10% solids. U.S. Pat. Nos. 6,454,102 (Thompson), 5,545,333 (Louden et. al.), 5,240,609 (Langley), 5,066,399 (Sugihara et al.), 5,022,989 (Put), 4,986,910 (Uyama et al.), 4,961,862 (Janecek), 4,836,100 (Johnson et al.) and 4,181,616 (Bahr) describe belt presses with gravity screening zones.
After the gravity screening zone, the mixture of solids and liquids enters the wedge zone where the upper belt and the lower belt carrying the mixture of solids and liquid converge with one another, applying progressively increasing pressure as the mixture is compressed between the belts. The degree to which the mixture is compressed corresponds to the angle of convergence of the belts, also called the wedge angle. The wedge angle may be fixed at all times or mechanically adjusted prior to running the filter press to accommodate a particular mixture. Wedge zones are shown in U.S. Pat. Nos. 7,381,329 (Moss), 6,454,102 (Thompson), 5,545,333 (Louden et al.), 5,240,609 (Langley), 5,109,764 (Kappel et. al), 5,066,399 (Sugihara et al.), 5,022,989 (Put), 4,986,910 (Uyama et al.), 4,961,862 (Janecek), 4,879,034 (Bastgen), 4,836,100 (Johnson et al.), 4,681,033 (Crandall et al.), 4,181,616 (Bahr), 4,053,419 (Pav), 3,942,433 (Wohlfarter), 3,894,486 (Sparowitz et al.) and 3,796,149 (Heissenberger).
Some belt presses have a higher pressure zone wherein the upper and lower belt follow a path between progressively smaller press rolls which may be either perforated or solid drums. The pressure imparted to the material between the upper and lower belts in the high pressure stage increases greatly from the largest perforated drum or solid roller to the smallest perforated drum or roller. High pressure zones are shown in U.S. Pat. Nos. 5,545,333 (Louden), 5,240,609 (Langley), 5,066,399 (Sugihara et. al), 4,986,910 (Uyama et al.), 4,879,034 (Bastgen), 4,836,100 (Johnson et al.), 4,681,033 (Crandall et al.), 4,181,616 (Bahr), 3,942,433 (Wohlfarter), and 3,796,149 (Heissenberger). Except for Uyama et al., each of these inventors teach us that gradually increasing compression pressure through the use of press rollers arranged in decreasing diameters in the direction of movement of the solids/liquid mixture corresponds to a progressive degree of liquid removal. Likewise, increasing shearing force through multiple changes in the direction of movement of the solids/liquid mixture by Z-shaped or S-shaped pathways also corresponds to a progressive degree of liquid removal.
The problem with all prior art is if the porous belt filter fabric becomes clogged or blinded, then liquid can not be separated from the solids/liquid mixture. In U.S. Pat. No. 4,986,910 (Uyama et al.), the inventors teach us that liquid in the solids/liquid mixture is removed only in the vertical direction through the filter fabric(s). To overcome the problem with blinding or clogging, the inventors formed pellets from pre-dehydrated sludge as an intermediate step prior to high compression pressing. The gaps between pellets that were then passed through the high compression zone provided a pathway for liquid to escape to the filter cloth and exit the solid/liquid mixture. The inventors claimed a higher removal rate of liquid in the high compression zone when forming pellets prior to high compression pressing
Another problem with the prior art practitioners whom rely on multiple press rollers of decreasing diameter and multiple changes of direction is described in U.S. Pat. No. 4,879,034 (Bastgen) who argues that pressure plates are superior to multiple press rollers because multiple press rollers have sections of reduced pressure between pinch points of multiple press rolls. Liquid that is pressed out in high compression zones at the pinch points will be sucked back into the solid/liquid mixture in low compression zones between the pinch points.
Therefore, the object of this invention is to provide a method for improving all belt press designs by either adding or enhancing the high compression zone using a superior method, the net result being to produce press cakes with higher solids content and less liquid.